A thermoelectric generator (TEG) converts heat (more precisely a heat flow passing through the thermoelectric generator) directly into electrical energy. In this respect, a thermoelectric generator is made from a plurality of thermocouple elements (often also called “thermoelectric modules”) (TEM) mounted between a cold side and a warm side of the thermoelectric generator and electrically connected in series and/or in parallel. The thermocouple elements effect the actual conversion of the thermal energy into electric energy. The conversion can be based on different effects (e.g., the Seebeck effect or the Thomson effect). For providing a thermal conductivity between the cold and the warm sides and the thermocouple elements, the thermoelectric generator often comprises one or more contact members made from a material with high thermal conductivity. Such contact members serve, in particular, to even out asperities and to thereby enable thermal conduction at the entire surface of the thermocouple elements.
The range of the operation temperature within which known thermocouple elements have a reasonable efficiency is relatively narrow. Within the operation temperature range, the efficiency of the thermocouple elements is defined by the temperature difference between the cold side and the warm side. The efficiency has inter alia an impact on the (electric) voltage provided by a thermocouple element. Common thermocouple elements are basically comprised of Bi2Te3, PbTe, SiGe, BiSb or FeSi2.
When using a thermoelectric generator, it is of importance to integrate the sensitive (in particular sensitive to pressure) thermocouple elements into the fluid flow (e.g. exhaust tract of a vehicle driven by an internal combustion engine) such that the maximum usable temperature difference is applied to surfaces of the thermocouple elements, while the back pressure for the fluid may not be increased or only slightly increased, because increase in the back pressure also results in an increase of the energy required for the transport of the fluid (and thus also of the fuel consumption of a vehicle driven by an internal combustion engine). Finally, it is desirable to operate the thermocouple elements, if possible, in an operation temperature range where they exhibit a reasonable efficiency. When used in vehicles, these problems are also difficult to solve due to the confined spaces in or below the vehicles.
A use of thermoelectric generators in exhaust systems of vehicles driven by an internal combustion engine is known from the prior art. For examples, it is referred to EP 1 475 532 A2, JP H07-012009 A, and WO 2009/138158 A1.
From WO 2009/138158 A1 it is known to cool the thermoelectric generators employed by coupling their cold side to a coolant circuit. Further, it is known from WO 2009/138158 A1 to increase the thermal absorption from the hot exhaust gas at the warm side of the employed thermoelectric generators using fins. The density of the fins or the type of fins employed should thereby vary along the direction of flow of an exhaust gas stream for achieving a lower thermal conductivity for thermocouple elements at an upstream region of the hot exhaust gas stream than for thermocouple elements at a downstream region of the hot exhaust gas stream.
A disadvantage of prior art systems is that often a number of thermoelectric generators configured for different operation temperature ranges are used in combination with bypasses for the warm fluid in order to allow a use of a broad temperature range of the warm fluid, when the temperature of the warm fluid varies. Bypasses, however, require a lot of space and are expensive, due to the plurality of thermoelectric generators employed.
Further, one of the problems with prior art systems is often that thermocouple elements of a thermoelectric generator that are arranged in tandem/in series along the direction of flow of the warm fluid experience different temperature differences due to the thermal energy being extracted from the warm fluid. This results in the thermocouple elements generating different electric voltages (and providing different output) accounting for a complex configuration of a connected power electronic.
In addition, prior art systems have the problem that any thermal overloading of thermocouple elements of a thermoelectric generator located upstream along the warm fluid is to be prevented.
Finally, fitting a thermoelectric generator is laborious due to the plurality of components (thermocouple elements and contact members).